Auxiliary liquid-holding tanks usually are designed to be filled with enough liquid to operate, for about a day, emergency apparatus, such as stand-by engine-generators for hospitals, schools, etc. The usual systems for filling such "day tanks" from large reservoirs at lower levels comprise power-driven pumps and conduits which encounter several reliability-reducing problems: namely, loss of prime in the pump due to back-flow or conduit leakage, aggravated by long quiescent periods; and functional failure of components, due to gumming or drying of bearing lubricants, and corrosion of bearings, linkages and electrical-contact surfaces, etc. Heretofore, the pumps have generally been located without regard to their elevation, often on the top of the tank, a position conducive to loss of prime. Further, the inlet to the tank was often located above the pump-turn-on level, an arrangement also conducive to loss of prime in the supply system. Moreover, prior-art day-tank systems have also generally lacked means for readily testing the operability of the various controls and indicators to insure their dependable operation.